Fix the rebound when swipe down slowly

When refresh state changes from PULL_TO_REFRESH to RELEASE_TO_REFRESH,
listview will rebound, so I changed the calculation of paddingTop.
What's more, after mRefreshViewImage becomes visible, mRefreshViewHeight
will change, set the default visibility of mRefreshViewImage to visible
will fix that bug.
Merge branch 'master' of github.com:cashow/android-pulltorefresh into cashow